Nitrogen monoxide acts as a catalyst in the oxidation of sulfur dioxide to sulfur trioxide according to the overall stoichiometry
SO2(g)+21O2(g)⟶SO3(g).
The following two-step sequence has been proposed:
1. NO+O2⇌NO3 (fast equilibrium)
2. NO3+SO2→NO+SO3 (slow)
Experimental measurements performed at 600 K give
rate=k[SO2][O2].
A student argues that the appearance of NO in the fast step implies that the catalyst concentration should enter the rate law. Evaluate this claim.
(a) Derive the rate law implied by the proposed two-step sequence, expressing the final rate law only in terms of the concentrations of species present in the overall stoichiometry.
(b) State whether the derived rate law agrees with the experimental rate law. Justify any agreement or disagreement.
(c) Explain, with reference to the role of a catalyst in elementary steps, why the experimental rate law does or does not contain the catalyst concentration.